I believe the answer is:
You would walk up to one of the trolls, point in the direction of the other and ask him: "Would that troll say that the door you are standing in front of is the heaven door?"
Whether or not, the troll you are talking to is the truth teller or the liar, the answer in this case:
If the troll you are talking to answers "No", then you are indeed talking to the troll that stands in front of the heaven door. If the troll you are talking to answers "Yes", then you are talking to the troll that stands in front of the door to hell.
If I am wrong, someone please correct me. For the love of all those people who read this before something ominous happens...